The influence of cross correlation on multiplet patterns in nuclear overhauser effect spectra

In the presence of cross correlation between the chemical-shift anisotropy and dipole-dipole relaxation mechanisms the individual lines of a weakly coupled multiplet can receive different NOE enhancements. The resulting multiplet asymmetry is due to the presence of antiphase terms which arise solely as a result of relaxation, rather than their more usual origin through selective population transfer effects. The enhancements measured from the overall integrated multiplet intensity are not substantially affected by cross correlation. Experimentally, the multiplet asymmetry is especially pronounced in three spin {H}19FH systems, when observed at high-field strengths. Similar effects are predicted for cross correlation between separate dipole-dipole relaxation pathways. The generation of these antiphase terms is related to the appearance of "forbidden" cross peaks in multiple-quantum-filtered COSY spectra. © 1987.